Food- and water-borne pathogens constitute a continuing health problem in human history, and inflammatory bowel disease in our society has an increasing trend. Thus, the study of host- microbe interaction and innate immune response in the intestine can provide novel strategies for therapy. This proposal aims at establishing a new genetic model system to study how the gut epithelium reacts to chemicals and microbes. Drosophila melanogaster, the common fruit fly, has emerged as a powerful tool for analyzing human disease genes and studying innate immunity. Drosophila innate immunity uses evolutionarily conserved mechanisms to fight infections. Indeed, the study of mammalian Toll-like receptors is originally based on the Drosophila Toll. Injection of microbes to Drosophila can induce systemic immune responses in fat bodies and hemocytes but feeding of microbes to wild type flies rarely causes lethality. The Drosophila gut epithelium acts as a strong barrier and has both constitutive and inducible antimicrobial defense. The constitutive defense involves reactive oxygen species (ROS) and annulment of this mechanism causes significant lethality when flies ingest common bacteria including E. coli. How Drosophila gut cells respond to microbes and mount an inducibe defense is not well understood. We demonstrate by gene expression profiling that the adult Drosophila gut has an extensive response after feeding with bacteria. We have also identified genetic mutants that have increased susceptibility to gut pathogens. Moreover, feeding of dextran sulfate sodium (DSS), a widely used chemical that can induce ulcerative colitis in mammals, causes dose dependent lethality, and co-ingestion of pathogenic bacteria enhances this induced lethality. We also observe that feeding of DSS causes pathological changes in the gut including abnormal intestinal stem cell behavior. Thus, Drosophila gut interacts with environmental stimuli and generates detectable responses. The specific aims of this proposal are to investigate the Drosophila gut response to microbes, to understand how DSS changes epithelial and stem cell behavior, and to perform a pilot genetic screen for host genes essential for self-defense in the gut. Data obtained from this R21 exploratory study will be used for future R01 grant submission and will provide novel diagnostic and therapeutic strategies to protect the public against pathogens and inflammatory diseases in the intestine. [unreadable] [unreadable] [unreadable]